New research shows that babies born as early as 22 weeks into gestation may be viable with aggressive medical intervention. A small fraction will survive, but most of those who do will have serious impairments. By 23 weeks, however, about a quarter of babies survive with medical help, the study shows. The debate over the number of weeks at which a fetus is considered to be viable reignites controversies over abortion, which, according to the Supreme Court, is allowed as long as the fetus is not viable outside the mother’s body. About 5,000 babies are born at 22 or 23 weeks each year. A full-term baby is one born at 39 to 40 weeks.